M20K TAS/IAS

[KASE]

Hi there.

I’m a relative newcomer to a 1982 K model with aftermarket waste gate and intercooler. Curious to find out what kind of true and indicated airspeeds K model pilots are typically experiencing.  I normally operate in the 11K to 18K altitude range, 70% power and ROP.  My indicated airspeeds are usually in the 120-128 range depending on density altitude-which makes for approximately 154-160 TAS calculations.  Seems a little slow to me. I was expecting faster true airspeeds. Appreciate hearing what others are experiencing.  Thanks in advance! 

[hubcap]

My K model is a 231 and has the Turbo Plus intercooler and the Merlyn Black Magic. I have found that 15k - 19k is really the sweet spot for optimum speed and fuel consumption. I typically file for 155kts at 15k and 165kts at 18k-19k. Those speeds are running LOP at ~65% power. I typically run about 30" MP and 2400 RPM.

Hopefully @jlunseth  will come along and comment as he is far more experienced and knowledgeable than I am with the 231.

[jlunseth]

How are you determining that you are running at 70% HP ROP?

If you have the Turboplus intercooler, it came with a chart. You would use the differential temperature, that is, the difference between Compressor Discharge Temperature (before the intercooler) and Induction Air Temperature (temperature after the intercooler). The POH would give you a power setting (factory configuration, without the intercooler) in order to operate at 70% power. The chart would then tell you what MP you would need to use, given the intercooler and the diff. temp, to operate at 70% HP. 

The other way of determining percent HP would be to have an engine monitor, mine is a JPI 930, that displays % HP.

It has been years since I operated the engine using either the chart or the %HP display, but back when I did, I always felt the plane was slow. I don’t recall the indicated airspeed, but it was in the high 120’s or low 130’s and I never got the airspeeds that the POH showed for a 70 or 75% power setting.

When I began to operate the engine LOP, one of the first things I learned (from the GAMI seminar) is that %HP while LOP is directly a function of fuel flow. The formula is 13.7 x fuel flow=HP produced, divided by 210 (the rated HP)=%HP. HP does not vary with MP so long as the engine is operating LOP. I developed a power setting that produced 70% HP while LOP and magically, my indicated airspeeds since then have been in the 138-142 range and the TASs at altitude have been correspondingly higher. 

Researching why the ROP results were so poor I learned a couple of things. First, there were several articles, which I verified with some engineer friends, that the turbocharger manufacturers, in creating those charts, were making the wrong assumptions that there is no cost (in terms of power) to reducing the temperature of the induction air by running it through the intercooler. Actually, pushing the induction air through the intercooler creates back pressure in the exhaust and causes the engine to work harder. It also affects the combustion cycle because of the backpressure. IT “robs” some HP.

My plane has been in the shop for several months getting a new engine (the old one was several hundreds hours over TBO), so I haven’t flown for awhile, and on top of that, I don’t fly ROP much anymore, so my memory about ROP power settings is poor, but my recollection is that at 13.5 GPH and 2450 RPMs, the charts say that the MP should be about 29” for 70%, but the actual setting that produces the same ROP airspeed as my known good 70% LOP setting is more like 31”. The “cost” in terms of MP to push the air through the intercooler is nearly as much as the charts would suggest is gained by dropping the induction temperature.

I should explain that. The gas laws say that for a given pressure, the number of molecules of a gas varies directly with the temperature, in other words, cooler air contains more O2 than warmer air, and the number of molecules of O2 in a given volume of induction air directly affects how much fuel can be burned. In other words, you can burn more fuel and produce more HP with cool air than with hot air. So when an intercooler drops the temperature of the induction air by, say, 125 dF, the same MP will burn more fuel and produce more power.

The engine monitor reading makes the same error as the charts. The JPI monitors use a proprietary algorithm and since it is proprietary, I don’t know for sure what it is. But after a few years of operating it, it is pretty obvious that JPI used the POH + intercooler chart settings. 

That does not mean that there is no gain from using an intercooler. There is quite a bit of gain. The cylinders run quite a bit cooler and there is some gain in the amount of O2 going into the cylinders because the induction air temperature is lowered. The gain is just not as great as the charts from the manufacturer says it is. The factory non intercooled engine had a CDT temp limit of 280 dF. Based on the temps I see in my engine, it is pretty clear that on most hot summer days, a non intercooled engine simply could not climb beyond about 17 or 18k without running into CDT issues, and those issues cause detonation. The cooling effect of the intercooler is enough that the CDT limit is never approached in the intercooled engine.

Now, I am just a pilot, I haven’t run an engine on a test stand to verify all this. I am just telling you what I learned from experts and engineers, and experienced in running my engine. However, its is well documented in the engine industry that the initial claims of gains from lowering the IAT were not as great as originally thought. And there is the fact that at 70% LOP using the accepted HP formula, my indicated airspeed is 8-10 kts faster than a 70% ROP setting based on the charts and POH.

Edited October 10, 2023 by jlunseth

[Z W]

It's a little slow.  I suspect you're not really putting out 70% power, or your airspeed indicator may not be accurate.  What MP / RPM / fuel flows are you using?  Have you verified the speed with a GPS speed check?

I get about 132-135 KIAS while ROP.  Settings are close to 27-28" MP, 2450 RPM, 12.5 GPH.  Speed varies a bit with the cowl flap positions which change based on altitude and outside air temp to keep the cylinder temps under 380.  Increasing to 13.5 GPH will get closer to 140 KIAS but at an obvious hit to efficiency. 

Reducing power to something like 26" MP, 2450 RPM, 10.5 GPH produces 125-128 KIAS for a higher efficiency cruise.  Used that on my last trip to stretch the final leg.

My engine does not run smooth LOP.  Your experience may vary.

Beware the internet - someone may be along shortly to post numbers at least 10% better than this in both speed and fuel burn.  But this has been my experience with a TSIO-360-MB in a K model.

[Pinecone]

8 minutes ago, Z W said:

Beware the internet - someone may be along shortly to post numbers at least 10% better than this in both speed and fuel burn.  But this has been my experience with a TSIO-360-MB in a K model.

Of course they will, this is a bunch of MOONEY pilots.

I have a 252 converted to Encore (-SB engine).  I run 63% power, LOP at 32.5/2300 at 10.1 GPH and get 130 - 135 IAS.  I normally fly in the 12K - 17K range and end up 170 - 174 KTAS.

[KASE]

Thanks all! 
 

So, I have been using the 70% power setting table for the intercooler which usually gives me a setting of around 25.5-26 inches at 2,500RPM/12.5 GPH. The JPI engine monitor does indicate very close to 70% once I get the throttle/prop./mixture dialed in and stabilized. 
 

I do have Gamijectors and have experimented with LOP several times, but find I lose more than 8 KTAS to save 1.75 GPH-not worth the trade off to me.

[KSMooniac]

1 hour ago, KASE said:

Thanks all! 
 

So, I have been using the 70% power setting table for the intercooler which usually gives me a setting of around 25.5-26 inches at 2,500RPM/12.5 GPH. The JPI engine monitor does indicate very close to 70% once I get the throttle/prop./mixture dialed in and stabilized. 
 

I do have Gamijectors and have experimented with LOP several times, but find I lose more than 8 KTAS to save 1.75 GPH-not worth the trade off to me.

Make sure you read jlunseth's excellent post again.  You might want to consider taking the www.advancedpilot.com seminar as well...that is the best way to learn all about this engine stuff.  With a turbo, you don't *have to* lose speed to go LOP, which might be the best part of owning a turbo!  In fact. you might find you can go even faster than a 70% ROP power setting, with a cooler engine and lower fuel burn.  Seriously.

[Bigdaddie]

On 10/9/2023 at 2:40 PM, hubcap said:

My K model is a 231 and has the Turbo Plus intercooler and the Merlyn Black Magic. I have found that 15k - 19k is really the sweet spot for optimum speed and fuel consumption. I typically file for 155kts at 15k and 165kts at 18k-19k. Those speeds are running LOP at ~65% power. I typically run about 30" MP and 2400 RPM.

Hopefully @jlunseth  will come along and comment as he is far more experienced and knowledgeable than I am with the 231.

These are almost identical to the numbers I get.  I also use about 30" and 2400RPM LOP with FF being in the 9.5-10 GPH range.

[Red Leader]

Wax the underside of the wings. Clean and waxed, you will notice a nice improvement in performance. 

[T. Peterson]

On 10/10/2023 at 6:02 AM, Z W said:

It's a little slow.  I suspect you're not really putting out 70% power, or your airspeed indicator may not be accurate.  What MP / RPM / fuel flows are you using?  Have you verified the speed with a GPS speed check?

I get about 132-135 KIAS while ROP.  Settings are close to 27-28" MP, 2450 RPM, 12.5 GPH.  Speed varies a bit with the cowl flap positions which change based on altitude and outside air temp to keep the cylinder temps under 380.  Increasing to 13.5 GPH will get closer to 140 KIAS but at an obvious hit to efficiency. 

Reducing power to something like 26" MP, 2450 RPM, 10.5 GPH produces 125-128 KIAS for a higher efficiency cruise.  Used that on my last trip to stretch the final leg.

My engine does not run smooth LOP.  Your experience may vary.

Beware the internet - someone may be along shortly to post numbers at least 10% better than this in both speed and fuel burn.  But this has been my experience with a TSIO-360-MB in a K model.

Your numbers are almost exactly what I get in my K. @jlunseth does significantly better but he had his engine dialed into perfection! I have no intercooler, no Gami injectors and no fine wire plugs, however Gami’s are on the horizon. Everyone I talk to is thoroughly impressed with their performance.

[T. Peterson]

9 hours ago, Bigdaddie said:

These are almost identical to the numbers I get.  I also use about 30" and 2400RPM LOP with FF being in the 9.5-10 GPH range.

Show offs!  

[Crawfish]

I only have about 7 hours in my Mooney now! Buutttt one of those flights was 4 hours literally 4:00, on that flight I set power at 2400 RPM 31” and leaned until rough and enriched just enough to smooth it out. At 6500’ it gave me 148TAS 8500 155TAS and 10500 160TAS and burned 12.3 GPH averaged over the whole trip which included a rejected Take off, and long taxi back.

 135IAS at all three altitudes temp was 4°c at all three altitudes 

 

I don’t know what my percent power was as the fuel flow gauge was bouncing between 9.5 and 11GPH at constant power. But I was lean of peak so somewhere in the 60-70 percent power range I’d venture to guess  

I want to play around with more power settings but planes going into the shop to get an full engine monitor. So hopefully only a week or so. 

[geoffb]

My -MB doesn't run smooth LOP either.  So, setting RPM and MP per the 65% power chart and leanest cylinder leaned to 80 ROP, which seems to be 12.0-12.2 gph mine is roughly 

150 true at 10k

165 true at 15K

170 true at 17k

up the power to 70% and add about 5 knots, go up to max cruise and add 5 more

This is a fat (2,100 lb) 262 conversion with TKS and prop boots, so it's relatively slow.  So for a clean 231, yours seems a little slow maybe.  But there's so many variables.

[Crawfish]

Got my fuel system tweaked (so it doesn’t bounce around so much) while waiting on a couple parts for the EIS And took it on another trip!

11500 60% power 152 TAS 

8500 63% 145 TAS 

both power values were LOP. 

Plane has a semi severe right turning tendency inflight so hopefully it will gain a few knots once that’s sorted out!

[Crawfish]

Little more data for anyone curious. M20K no intercooler no merlyn   

4+29 hour take off to touchdown 

  16,000’ started at 165KTAS finished 175KTAS Total trip burn was 50.5 gallons. Was leaned to the mid 10GPH range in cruise. 
 

4+36 T/O to Touchdown 

  13,000’  155-160 KTAS. Total leg burn 51.7 gallons, same leaned to mid 10GPH range in cruise. 
 

Fuel burn was measured at the pumps by topping the plane back off. Reason I say mid 10GPH range is my fuel flow bounces a little bit in cruise .3 of a gallon or so. 

[LANCECASPER]

2 hours ago, Crawfish said:

Little more data for anyone curious. M20K no intercooler no merlyn   

4+29 hour take off to touchdown 

  16,000’ started at 165KTAS finished 175KTAS Total trip burn was 50.5 gallons. Was leaned to the mid 10GPH range in cruise. 
 

4+36 T/O to Touchdown 

  13,000’  155-160 KTAS. Total leg burn 51.7 gallons, same leaned to mid 10GPH range in cruise. 
 

Fuel burn was measured at the pumps by topping the plane back off. Reason I say mid 10GPH range is my fuel flow bounces a little bit in cruise .3 of a gallon or so. 

What was your TIT and your CHTs?

[Crawfish]

@LANCECASPER

CHT’s were somewhere in the mid to high 300’s (still rocking the stock CHT gauge, GI275 scheduled for install in January)  

TIT were sitting between 1530-1550. At 16,000

about 1500 at 13,000
 

Not sure how far LOP I was, as I do the “big pull” then verify I’m lean of peak by enriching to make sure TIT rises then pulling back to where I wanted to be at.